The present invention relates to a MEMS transducer for interacting with a volume flow of a fluid, e.g., a MEMS loudspeaker, a MEMS microphone or a MEMS pump. Furthermore, the present invention relates to a method for manufacturing a MEMS transducer. Furthermore, the present invention relates to a MEMS-based electroacoustic transducer.
In addition to the miniaturization, one focus of the MEMS technology (MEMS—microelectromechanical system) particularly lies in the potential for a cost-effective manufacturability of the component in medium and high quantities. Electroacoustic MEMS loudspeakers are currently not significantly commercialized. With few exceptions, MEMS loudspeakers consist of a membrane which is deflected quasi-statically or resonantly by a selected physical operating principle. The deflection linearly or non-linearly depends on the applied electric signal (current or voltage). The signal comprises a temporal variation which is transmitted into a temporal variation of the membrane deflection. The reciprocating movement of the membrane is transmitted in the form of sound into the surrounding fluid which, for the sake of simplification but not for limitation, is assumed to be air.
In some cases, the actuation of the membrane occurs only in one direction. The restoring force is then provided by the mechanical spring action when the membrane is deflected. In other cases, the actuation occurs in both directions so that the membrane may comprise a very low rigidity.
For the actuation of the membrane, the use of electrostatic, piezoelectric, electromagnetic, electrodynamic and magnetostrictive operating principles is described. For example, an overview of MEMS sound transducers based on these principles may be found in [1].
Electrostatically operated transducers are based on the force which results between two flat electrodes engaged with different electric potentials. In the simplest case, the arrangement corresponds to a plate capacitor, wherein one of the plates is movably suspended. In practice, the moveable electrode is embodied as a membrane in order to avoid an acoustic short circuit. When applying a voltage, the membrane buckles in the direction of the counter electrode. In a specific embodiment, the membrane is operated in the so-called touch-mode. In this case, the membrane touches the lower electrode onto which a thin insulator layer is applied in order to avoid a short circuit, e.g., as described in [2]. In this case, the contact area is determined by the size of the electric voltage applied and, thus, varies temporally according to the temporal course of this voltage. The oscillation which may be generated in this way serves for generating sound. In the classical electrostatic structure, the membrane may in principle only be attracted in the direction of the electrode. The restoring force may be determined at least partially by the rigidity of the membrane and has to be sufficiently high in order to be able to also transmit the higher frequencies in the auditory sound range.
On the other hand, when an electric voltage is given, the deflection of the membrane may decrease with an increasing rigidity. In order to avoid this problem, an approach with a very soft membrane has been developed, which may be driven by an upper and a lower electrode and, thus, may be deflected in both directions, as described in [3]. In total, this loudspeaker uses two of such membranes suspended in the interior of a cavity comprising, same as in a micropump, an inlet and an outlet and otherwise being closed.
Piezoelectrically operated transducers use the inverse piezoelectric effect. An applied electric voltage leads to mechanical stress in a solid body. In the MEMS technology, materials such as PZT (lead zirconate titanate), AlN (aluminum nitride) or ZnO (zink oxide) are typically used. Usually, these materials are applied as a functional layer onto a membrane and are structured such that the membrane may be deflected, or excited to oscillate, depending on the electric voltage applied at the functional layer. A disadvantage of piezoelectric functional layers is the fact that the operation may not be performed without hysteresis. Furthermore, the integration of the ceramic functional layers is complex and, due to the lack of CMOS compatibility (CMOS=complementary metal oxide semiconductor), only possible under strict contamination control or in a separate clean room area when using PZT and ZnO.
Electromagnetically operated transducers are based on the force effect that a soft magnetic material is subjected to in a mobile magnetic field (gradient). Implementing the principle, besides the soft magnetic material, involves a permanent magnet and a coil by means of which the local gradient of the magnetic field may be temporally controlled via a current flow. For example, the soft magnetic material is integrated into the membrane. All other components are provided during the assembly, e.g., as described in [4]. The structure is voluminous, complex and does not seem to be scalable in a meaningful way with respect to large quantities.
Electrodynamically operated transducers use the Lorentz force. This method is very widespread in macroscopic loudspeakers and has also been used in some MEMS loudspeakers. The magnetic field is generated by a permanent magnet. A current-carrying coil is placed in the magnetic field. Usually, the coil is integrated into the membrane by depositing and structuring a metal layer, and a permanent magnet is added as an external component during assembly The complexity and the limitations with respect to the integration of all components using the MEMS technology are a similarly large disadvantage as in the electromagnetically operated transducers.
Magnetostrictively operated transducers are based on a contraction or expansion of a functional layer when a magnetic field is applied. For example, Vanadium Permendur is positively magnetostrictive, i.e., expands when a magnetic field is applied. In a suitable structure, this contraction may be used for generating a membrane oscillation. In [1], Vanadium Permendur (Fe49Co49V2) deposited onto SiO2 (silicon dioxide) via a chromium adhesive layer is used as magnetostrictive functional layer. The external magnetic field is provided by a micro-flat coil realized by galvanically deposited copper. With respect to complexity and limitations of the integration, similar disadvantages are to be noted as with both above-mentioned operating principles.
The above-described classical and most widely used variations, which have as a common feature the use of a membrane which may be excited to oscillate, are subsequently supplemented by certain modifications which were investigated due to special disadvantages of the classical membrane principle.
Flexible membranes may also comprise higher modes in the auditory sound range and, thus, may lead to parasitic oscillations decreasing the acoustic quality (distortion factor), cf., [1]. Thus, in order to avoid or reduce this effect, plates comprising a significantly higher rigidity are used. Such a plate is connected to the chip via a very soft suspension which is to also avoid the acoustic short circuit, cf., [5].
Another modification provides a segmented membrane used with the above-described magnetostrictive transducers. This corresponds to a special topographical solution to the problem that the functional layer contracts or expands in two directions. Specifically, the structure consists of several deflectable bending bars. According to [1], the arrangement may be considered to be acoustically closed for distances of the bars smaller than or equal to 3 μm. By accordingly dimensioning the individual bars with respect to a resonance frequency and the distances between the bars, a comparatively high acoustic bandwidth may be achieved and the course of the sound level may be adapted or optimized as a function of the oscillation frequency.
In [6], Neumann et al. pursue the approach of using a multitude of small submembranes instead of a single large membrane. Each submembrane comprises a resonance frequency high enough so that a quasi-static deflection may occur in the auditory sound range. In particular, this enables a digital operation of the loudspeaker.
In summary, it may be concluded that, with respect to the integration, known electrostatically operated membrane loudspeakers comprise relatively small deflections when assuming moderate drive voltages. For example, the electrostatic membrane loudspeaker of Kim et al. according to [3] may serve as a reference. Each of the two membranes comprises an area of 2×2 mm2. The upper and lower electrodes are respectively attached at a distance of 7.5 μm. Depending on the geometry of the membrane and the increase in membrane rigidity with increasing deflection, the deflection is typically limited to ⅓ to ½ of the electrode distance due to the so-called pull-in effect. Assuming the higher value of ½, the deflection results in 7.5 μm/2, in one direction and in the other direction, respectively. The displaced volume may be estimated by assuming that it corresponds to the volume of a deflected rigid plate having the deflection of half of the maximum deflection of the membrane. For example, this results in:ΔV≈(2×2 mm2)×50%*(2×7.5 μm)/2=15×10−3 mm3  (Eq. 1)orΔV/active area=ΔV/A=ΔV/4 mm2=3.75×10−3 mm  (Eq. 2)
When manufacturing miniaturized membrane loudspeakers, it is a general problem to achieve a flat course of the sound pressure as a function of the frequency. The achievable sound pressure is proportional to the radiation impedance and the speed of the membrane. With respect to the macroscopic scale, the membrane diameter is comparable to the acoustic wavelength. What applies in this regard is that the radiation impedance is proportional to the frequency, cf., [6]. Often, high-quality loudspeakers are designed so that the resonance f0 is below the auditory sound range (for multi-way loudspeakers, the respective resonance frequency is below the lower edge frequency of the corresponding electric filter). Thus, for f »f0, the speed of the membrane is proportional to 1/f. Overall, the expression p ∝1 results for the frequency dependency of the sound pressure p. Thus, a completely flat course of the sound pressure curve results in this (simplified) consideration.
Once the diameter of the sound source/of the membrane is much smaller than the sound wavelength to be generated, a quadratic dependency from the frequency may be assumed for the radiation impedance, as described in [7]. This is given for MEMS loudspeakers having membranes in the magnitude of millimeters. Assuming f»f0, as above, the dependency p ∝f results for the course of the sound pressure curve. Low frequencies are reproduced with too low of a sound pressure in relation to the high frequencies. In the quasi-static operation, the membrane speed is proportional to f. Thus, for the sound pressure course, the dependency p∝f3 results, which is even more unfavorable for low frequencies.
Thus, a concept for improved MEMS transducers comprising a high degree of efficiency would be desirable.